Chemical strengthening of glass, also called ion-exchange strengthening or chemical tempering, is a technique to strengthen a prepared glass article by increasing compression within the glass itself. It generally involves introducing larger alkali ions into the glass chemical structure, to replace smaller alkali ions present in the structure. A common implementation of chemical strengthening in glass occurs through the exchange of sodium ions, having a relatively smaller ionic radius, with potassium ions, having a relatively larger ionic radius by submerging a glass substrate containing sodium ions in a bath containing molten potassium salts.
Chemical strengthening is often utilized to increase compression in order to increase strength, abrasion resistance, and/or thermal shock resistance into a glass article. The increased compression can be introduced to various depths in the glass and is often implemented within a surface layer. Chemical strengthening is commonly utilized for treating flat glass. But it may also be used for treating non-flat glass articles, such as cylinders and other shapes of greater geometric complexity.
Flat glass is commonly manufactured by a number of known techniques. These include the float glass method and drawing methods, such as the fusion down-draw method and the slot draw method. However, a prepared flat glass article may have variations in its chemical composition and/or structure at different locations in the glass. For example, flat glass that is manufactured by the float glass technique is often prepared by spreading softened glass material on a molten metal surface such as tin. The glass is then cooled to form a solid, flat glass. As a result, the prepared flat glass often contains a greater amount of tin on the side that was nearer the molten tin and the concentration of tin is commonly greater near the surface of that side.
Chemical strengthening is often used to treat glass having variations in chemical composition and/or structure at different locations in the glass. The variations produce locations that are treatment-rich or treatment-poor relative to each other for ion exchange and/or compression development in chemical strengthening. When chemical strengthening is used to treat such glass, the introduced compressive stress is often not uniformly distributed. This may introduce a bending moment and subsequent induced curvature in a glass article treated by chemical strengthening, particularly for glass articles having a width of less than 3 mm. The induced curvature is often undesirable. Induced curvature is especially problematic in manufacturing thin flat glass articles according to manufacturing specifications that include the enhanced physical properties associated with chemical strengthening, but without induced curvature. For example, glass used in manufactured electronic articles, such as displays for “smart” phones, often requires glass that is uniformly flat and high in strength and in abrasion resistance.
For a thin, flat glass article, such as an article having two major surfaces, the non-equivalence of interdiffusion of invading alkali ions and/or compression generation properties between the major surfaces of the flat glass substrate after chemical strengthening commonly often has an effect, such that a local force times the distance from the mid-plane of a glass article is not equivalent when summed from the treatment-poor surface to the mid-plane and from the treatment-rich surface to the mid-plane. Thus the net bending moment about the mid-plane is non-zero (i.e., there is a non-zero net bending moment of the stress about the mid-plane). As a result, bending stresses are generated. For glass articles of thin cross-section, these bending stresses generate deflection of the glass article from flat. That is, thin, chemically strengthened glasses manufactured by the float process often exhibit measurable curvature after chemical strengthening. The direction of curvature is often concave on the poor surface and convex on the rich surface.
In recent years, various types of efforts have attempted to overcome the problem of induced curvature that is associated with the chemical strengthening of glass. One approach involves grinding and polishing a prepared glass prior to chemical strengthening. The grinding and polishing is performed to remove those parts of a glass having a different chemical composition and/or structure. An example of this approach is grinding and polishing a flat glass made by the float method to remove the surface layer(s) containing a significant amount of tin. However, grinding and polishing the float glass introduces abrasions and may introduce other physical defects, in addition to the added time and expense associated with performing the grinding and polishing. Other approaches have involved secondary chemical treatments of prepared glass done prior to chemical strengthening. The secondary chemical treatments are utilized in an attempt to address differences chemical composition and/or structure at different locations in the glass. However secondary chemical treatments can alter the physical properties of the glass and otherwise degrade a glass produced through subsequent chemical strengthening. Also, like grinding and polishing, secondary chemical treatments involve the time and expense of an extra processing step that is done prior to chemical strengthening.
Given the foregoing, chemically strengthened glass and methods for making chemically strengthened glass are desired in which the strengthened glass has reduced induced curvature. It is also desired that the strengthened glass not have the drawbacks associated with grinding and polishing or secondary chemical treatment(s) applied in prior methods associated with the chemical strengthening of the glass. It is also desired that the strengthened glass have the improved physical properties of chemically strengthened glass, such as higher strength, higher abrasion resistance, and/or higher thermal shock resistance.